Direct access storage devices (DASD) have become part of everyday life, and as such, expectations and demands continually increase for greater speed for manipulating data and for holding larger amounts of data. To meet these demands for increased performance, the mechanical assembly in a DASD device, specifically the Hard Disk Drive (HDD) has undergone many changes.
The amount of data that can be stored on a disk is governed by many well-known physical principles. One factor in determining the amount of data that can be stored in an HDD is the ability of the magnetic transducer to write closely spaced data tracks onto the disk surface. The spacing of tracks on a disk surface is known as track pitch, and the unit of measure for expressing the density of tracks on a disk is tracks per inch or TPI. A smaller track pitch results in a higher TPI. The servo system of the HDD enables in part the magnetic transducer to be precisely positioned at a radius on the disk to write a data track, enabling adjacent data tracks to be written with a high TPI.
Coded information is written at every radius on a disk where a data track will be written. This coded information is used to identify the location of data to be written on the magnetic recording media of the disk. Once the data is written, the coded information is used to locate the written data so that it can be read. This coded information is known as servo data. Servo data identifies the location of the track with respect to its order of radius from the disk's center of rotation and its sector in a circumferential distance from an index around the disk. The process of writing servo data is known in the art as servowrite.
The majority of the servowrite processes used today comprise a magnetic transducer writing a servo track at each sector of each radius of a potential data track. The servowrite process is typically a time consuming process and depending upon the design of the HDD can require several hours per HDD to accomplish. Today's demands for higher TPI increase the time required for servowrite. For various reasons such as mechanical vibrations, thermal transients, and other such disturbances that move the magnetic transducer in an undesirable manner, servo data can have poor quality. All HDDs have their own criteria for the allowable quantity, density, and distribution of poor quality servo sites. These criteria are dependent upon the HDD design and manufacturer. In today's art of servowriting, if a servo poor quality criterion is exceeded, repairing servo poor quality requires that all servo data be erased and the servowrite process restarted. A servo poor quality criterion determines if a servo track is defective.